Optical imaging systems typically comprise a detector, usually placed within a housing having an aperture through which the detector may view an external scene, and a telescope that images the external scene. The telescope projects a high quality image of predetermined magnification onto the detector. Certain optical imaging applications require the ability to position the telescope in a constrained volume or cross section. Accordingly, for many applications, the telescope must be as compact as possible, while remaining consistent with the required optical performance for high quality imaging. Since the diffraction limited imaging resolution is limited by the aperture size of the imaging system, in many missile system applications the aperture consumes a very large fraction of the frontal cross-section of the missile system in order to achieve the desired imaging resolution.
Some conventional telescopes have used off-axis optical system arrangements. However, such off-axis designs generally do not maximize the available aperture size, and unnecessarily increase the cost of the telescope system. Other known configurations that maximize the use of the fraction of missile cross-section that is available for the optical system include obscured on-axis telescopes. While these configurations may better use the shape and volume available in the missile system, they adversely affect optical performance as a result of diffraction effects caused by the secondary mirror and secondary mirror support structures of the on-axis system.